Swept-parameter instruments such as oscilloscopes display a graph of a measured quantity on a CRT or similar display device. The displayed measurement trace will typically exhibit noise that can be divided into trace noise and/or noise floor errors. The noise floor is the noise displayed when the measured signal has an amplitude of zero. The trace noise is the noise that depends on the amplitude and/or phase of the measured signal.
There are currently three methods used for noise reduction in such instrumentation. The first class of instruments reduces the noise errors by reducing the resolution bandwidth of the instrument. The resolution bandwidth is the equivalent noise bandwidth of an instrument. By reducing the resolution bandwidth, less noise is detected along with the signal of interest, resulting in less trace noise and a lower noise floor. While this method provides a more accurate measurement, the measurement speed is decreased due to increased signal processing time if the bandwidth reduction is implemented digitally. In addition, the instrument's transient response time is also reduced
The second prior art method averages a number of traces together to reduce the noise errors. By averaging successive traces of data, measurement error due to noise is reduced. Averaging successive traces is equivalent to reducing the resolution bandwidth of the instrument using digital signal processing. While averaging results in more accurate measurements, the measurement speed is decreased due to the number of sweeps that must be taken.
The third method utilizes video filtering and smoothing techniques to reduce the noise displayed by the instrument. The video bandwidth is the equivalent bandwidth of the display trace process. Video filtering trace noise reduces the noise uncertainty. While video filtering is computationally efficient, it has two main problems. First, low pass filtering of the video signal smears signal details, and hence, introduces errors in those regions in which the signal is rapidly changing. Second, many instruments display the logarithm of the measured signal amplitude to provide a more useful display for signals that have large dynamic ranges. Prior art video filtering systems introduce errors in such displays near the noise floor.
Broadly, it is the object of the present invention to provide an improved filtering system and method for reducing noise errors in trace measurement instruments.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.